class_docstr: Broyden-Fletcher-Goldfarb-Shanno (BFGS) Hessian update strategy.
  
  Parameters
  ----------
  exception_strategy : {'skip_update', 'damp_update'}, optional
      Define how to proceed when the curvature condition is violated.
      Set it to 'skip_update' to just skip the update. Or, alternatively,
      set it to 'damp_update' to interpolate between the actual BFGS
      result and the unmodified matrix. Both exceptions strategies
      are explained  in [1]_, p.536-537.
  min_curvature : float
      This number, scaled by a normalization factor, defines the
      minimum curvature ``dot(delta_grad, delta_x)`` allowed to go
      unaffected by the exception strategy. By default is equal to
      1e-8 when ``exception_strategy = 'skip_update'`` and equal
      to 0.2 when ``exception_strategy = 'damp_update'``.
  init_scale : {float, 'auto'}
      Matrix scale at first iteration. At the first
      iteration the Hessian matrix or its inverse will be initialized
      with ``init_scale*np.eye(n)``, where ``n`` is the problem dimension.
      Set it to 'auto' in order to use an automatic heuristic for choosing
      the initial scale. The heuristic is described in [1]_, p.143.
      By default uses 'auto'.
  
  Notes
  -----
  The update is based on the description in [1]_, p.140.
  
  References
  ----------
  .. [1] Nocedal, Jorge, and Stephen J. Wright. "Numerical optimization"
         Second Edition (2006).
code: def __init__(
      self,
      exception_strategy="skip_update",
      min_curvature=None,
      init_scale="auto",
  ):
      if exception_strategy == "skip_update":
          if min_curvature is not None:
              self.min_curvature = min_curvature
          else:
              self.min_curvature = 1e-8
      elif exception_strategy == "damp_update":
          if min_curvature is not None:
              self.min_curvature = min_curvature
          else:
              self.min_curvature = 0.2
      else:
          raise ValueError(
              "`exception_strategy` must be 'skip_update' "
              "or 'damp_update'."
          )
  
      super(BFGS, self).__init__(init_scale)
      self.exception_strategy = exception_strategy
  
imports: from __future__ import division, print_function, absolute_import
  import numpy
  from numpy.linalg import norm
  from scipy.linalg import get_blas_funcs
  from warnings import warn
label_desc: Use image node_id 1 to create a new BFGS object from inherited base classes: FullHessianUpdateStrategy with example: obj = BFGS(exception_strategy, min_curvature, init_scale)